The U.S. abortion rate began to decline again from 2008 to 2011, a new study says. But the researchers say there’s no evidence that the new crop of state laws intended to restrict access to abortions is the source of the drop.

An estimated 1.1 million abortions were performed in 2011, a rate of 16.9 per 1,000 women ages 15 to 44, according to the Guttmacher Institute, a reproductive-health think tank that has conducted a periodic survey on abortion since the 1970s.

That’s the lowest rate since 1973, when the Supreme Court’s decision on Roe v. Wade made abortions legal in all states. Both the abortion rate and the number of abortions fell 13% between 2008 and 2011.

The study also found an increase in medical, rather than surgical, abortions. An estimated 23% of all abortions performed outside of hospitals were nonsurgical, up from 17% in 2008.

Between 2008 and 2010, 18 states passed 44 laws pertaining to abortion, according to the study, which claimed that such restrictions on abortion seem to have had little effect on the drop. “While most of the new laws were enacted in states in the Midwest and the South, abortion incidence declined in all regions,” the study says. Even states like New York, California and New Jersey that allow Medicaid payments for abortions for low-income women saw declines in abortion rates comparable to or even greater than the national decline.

Researchers believe the slow economy may have contributed to the decline: the birthrate also fell by 9% in that same time period. The other explanation is the growing use of long-acting contraceptives like the IUD, which can be more effective at preventing pregnancy than other contraceptive methods. Use of long-acting contraceptive methods increased from 4% to 11% during the study period.

Pro-life groups have already criticized the study. In a statement, Charmaine Yoest, president and CEO of Americans United for Life, called the report “an abortion-industry propaganda piece short on data and long on strained conclusions” and noted that all the reporting of data is voluntary.